This invention relates to a monitoring system suitable for use in checking the operating conditions of a screw compressor.
In checking the operating conditions of a screw compressor, it has been usual practice to use a plurality of pressure switches and temperature switches for indicating the quantities of state of pressure and temperature prevailing in various parts of the screw compressor during operation and compare the results obtained with values set beforehand for the respective switches, to thereby indicate failures of the screw compressor by lighting corresponding lamps provided in a number equal to that of the switches.
Meanwhile, in a screw compressor, volume control is effected in various ways for adjusting the flow rate and pressure of the air discharged therefrom. Typical of such volume control are on-off control and suction throttle control. The former consists, as disclosed in Japanese Patent Laid-Open No. 124698/81, for example, in opening and closing a suction throttle valve located on the suction side of the screw compressor in accordance with a discharge pressure, and the latter consists in continuously throttling the suction throttle valve in accordance with a discharge pressure. Particularly in on-off control, there are great differences in the pressure and temperature of the compressed air between the open (on-load) condition of the suction throttle valve and the closed (unloaded) condition thereof. The volume of the compressed air undergoes a large variation immediately after the valve is opened and closed. This variation is not constant and may vary depending on the conditions of operation of the compressor and the conditions under which the compressor is used, so that quantities of state will be transiently obtained which are either very larger or smaller than those obtained in steady state operation. Thus, when the monitoring system of the prior art used for checking operating conditions of the screw compressor relies on comparison of the current values of quantities of state of pressure and temperature obtained by pressure and temperature switches with values set beforehand for the respective switches, it would be impossible to carry out diagnosis of the conditions of the screw compressor with a high degree of accuracy and precision. This is because of the fact that in this monitoring system it is impossible to provide malfunction sensing switches capable of functioning effectively by coping with both the on-load condition and the unloaded condition, so that it is necessary to render inoperative malfunction sensing switches in unloaded condition which are set to function in the on-load condition. This makes the monitoring system unable to check some quantities of state in unloaded condition. The reverse may be the case. Moreover, it is difficult to predict or measure accurately in what manner the quantities of state of a compressor will undergo changes in a transient state. Thus, it is necessary to render the malfunction sensing switches inoperative in the transient state or set the values for the switches in a manner to be set apart by a sufficiently large margin to avoid misoperation. Thus, the aforesaid type of monitoring system for checking the operation conditions of a screw compressor suffers the disadvantage that accurate diagnosis of the operating condition of the compressor cannot be made in both on-load and unloaded conditions.
An object of this invention is to provide a system capable of monitoring the operating conditions of a screw compressor in an appropriate fashion.
Another object is to provide a system capable of monitoring the operating conditions of a screw compressor thoroughly.
Still another object is to provide a monitoring system for checking the operating conditions of a screw compressor capable of producing results that are highly reliable.
To accomplish the aforesaid objects, the invention provides a monitoring system for checking the operating conditions of a screw compressor comprising a suction throttle valve located on the suction side of the screw compressor and an air discharge valve located on the discharge side thereof for effecting on-off control of the flow rate of the discharged fluid by opening and closing these valves, with the monitoring system comprising a first pressure sensor for sensing the suction pressure of the screw compressor; a second pressure sensor for sensing the discharge pressure of the screw compressor; a temperature sensor for sensing the temperature of compressed air on the discharge side of the screw compressor; diagnosing and judging means supplied thereto with on-load and unloaded operational signals for respectively judging the operating conditions of components of the screw compressor in the on-load and unloaded conditions by comparing signals produced by the sensors with preset values; and indicating means for indicating the judgement passed by the diagnosing and judging means.
Additional and other objects, features and advantages of the invention will become apparent from the description set forth hereinafter when considered in conjunction with the accompanying drawings.